TOPIC: Quotas: What am I missing??

I'm working on creating a survey for my university to allow people to register for a bus that would take them to and from the local airports, bus terminals, etc.

I have fields requesting their personal info, then two radio list questions, one outbound, one inbound, asking them what their destinations are.

The questions have the same basic answers. The first three options for each of those questions is a destination. There is also a fourth option on each question, stating that the person will not be riding the bus in this direction (for example, needs a ride TO the airport on a friday, but will not be utilizing the sunday return bus).

There are only 25 seats on each bus, and I would like to prevent people from being able to sign up for a seat that is not there. I would, however, like them to still be able to sign up for the other bus if space is remaining.